•• • * I vMassive Mi UnderwayBy BERT W. OKULEYSAIGON UP I - The V. S. command announced today that20.000 U.S. and South Vietnamese troops had reoccupied1.000 square miles of the northwestern corner of South Vietnam In a major offensive that took American soldiers to the border of Laos.The command thus lifted the tightest news embargo of the Indochina war on the massive operation that began six days ago. It Insisted no American ground troops would be sentInto Laos but left open thequestion of an Invasion bySouth Vietnamese troops.It was announced at the same time that 12,500 South Vietnamese troops had crossed into Cambodia today In two-new operations, one In the Parrot's Beak sector and the other In the lower Fishhook region where last spring's Allied Invasion was centered. This brought to.21,000 the number of South Vietnamese fighting in Cambodia.SEPONE KEY OBJECTIVEVeteran UPI correspondent Leon Daniel reported from the old Green Beret camp at Lang Vel, a mile and a half from thelitary Move'i' * ;On Borderborder, that the U.S. positions would be untenable unless supported by troops Inside Laos and that the men Involved were told they would be there at least three months.Daniel said the object of the operation -the biggest In South Vietnam In three years - was to cut the Ho Chi Minh. trail, and that to do so effectivelymeant the capture of Sepone,’ 25 miles inside Laos. The mountain passes leading from North Vietnam empty into the Sep'6ne area, from which sipplies are sent southward. f .. The drive was supported bymassive U.S. air strikes into/ Laos,an offense by. B52s and I fighter-bombers that entered its 118th. day today. U;§# helicopter gunshlps and supply planes also were involved.The U.S. command said the drive had encountered little resistance as the Allies reoc-cupi^d a string, of bases already named Khe Sanh, . the Rockplle, Camp Carroll, Cam Lo, Lang Vel-all of them Just below, the demilitarized zone DMZ. vA spokesman said total casualties so far were three Americans wounded and four